So with rsyslog <> plugin, that would just a fallback if you need it forsome reason (e.g. run a v3 engine with a v4 plugin WITHOUT the need tobackport it [requires a v3 and v4 runtime to be present on the system,though].

For non-rsyslog syslogd's I expect that pipe is always used, because Ido not think they'll change their environment to adopt to the runtime.

rsyslog will provide wrappers for either interface. They will come asseparate binaries. There will be an input and output plugin to allow anyprocess (not necessarily rsyslog technology) to utilize a standard unixpipe interface for producing and consuming messages.